← MES Jr. Grade
Past Questions — 2012
Magnetic flux density at a point distance R due to an infinitely long linear conductor carrying a current I is given by
A)
B = I / 2πR
B)
B = μI / R
C)
B = μI / 2πR
D)
B = μI / 4πR
For an infinitely long straight current-carrying conductor, Ampere’s law gives the magnetic field intensity H = I / 2πR. Since magnetic flux density B = μH, we get B = μI / 2πR. Option (a) is missing the permeability μ, while (b) and (d) have incorrect coefficients.
An n-bit AD converter using VR as reference voltage has a resolution (in volts) of
A)
VR/2n
B)
VR/n
C)
VR/2n-1
D)
VR/2^n
For an n-bit ADC, the total input range is divided into 2^n discrete levels. Hence the resolution (least significant step size) is reference voltage divided by 2^n, i.e., VR/2^n. Among the options, this is written as VR/2n in option (a), which is the intended answer. Option (b) is incorrect because resolution does not depend linearly on n, and option (c) corresponds to a different expression sometimes used for full-scale step count, not the standard resolution formula.
An e.m.f. induced in a coil due to a change in current in a neighbouring coil is known as
A)
self-induced e.m.f.
B)
speed e.m.f.
C)
mutually induced e.m.f.
D)
none of these
When the current in one coil changes, the changing magnetic flux links a nearby coil and induces an e.m.f. in it. This is called mutually induced e.m.f. Self-induced e.m.f. occurs in the same coil due to its own changing current. Speed e.m.f. is induced due to motion of a conductor in a magnetic field.
A Wheatstone bridge cannot be used for precision measurements because errors are introduced on account of
A)
resistance of connecting leads
B)
thermo-electric emfs
C)
contact resistance
D)
all of the above
A Wheatstone bridge is affected by several practical errors: resistance of connecting leads, thermo-electric emfs at junctions, and contact resistance in connections and keys. These introduce measurement errors and reduce precision. Hence, the correct choice is 'all of the above'.
Creeping in a single-phase induction type energy meter is due to
A)
overcompensation for friction
B)
overvoltage
C)
vibrations
D)
all of the above
Creeping in a single-phase induction energy meter means the disc rotates slowly even when there is no load. It can be caused by overcompensation for friction, overvoltage, and vibrations. Hence, all the given factors can lead to creeping.
The electric circuit equivalent of flux in a magnetic circuit is
A)
load resistance
B)
current
C)
voltage
D)
conductivity
In the magnetic circuit–electric circuit analogy, magnetic flux corresponds to electric current. Similarly, magnetomotive force (MMF) corresponds to voltage, and reluctance corresponds to resistance. Therefore, the electric circuit equivalent of flux is current. The other options are incorrect because load resistance and conductivity are not the direct analogues of flux, and voltage corresponds to MMF, not flux.
Inside a hollow conducting sphere
A)
electric field is zero
B)
electric field is a non-zero constant
C)
electric field changes with the magnitude of the charge applied to the conductor
D)
electric field changes with distance from the centre of the sphere
For a hollow conducting sphere in electrostatic equilibrium, all excess charge resides on the outer surface. The electric field inside the conducting material and inside the hollow cavity is zero, provided there is no charge placed inside the cavity. Hence the correct option is (a). Options (b), (c), and (d) are incorrect because the field inside does not depend on charge magnitude or distance from the centre; it remains zero everywhere inside.
Piezo-electric transducers are
A)
passive transducers
B)
active transducer
C)
inverse transducers
D)
both (b) and (c)
Piezo-electric transducers generate an electrical output directly from applied mechanical stress, so they do not need an external power source. Hence, they are **active transducers**. They also work on the inverse piezoelectric effect in the sense that mechanical energy is converted into electrical energy, so they are often referred to as **inverse transducers**. Therefore, the correct choice is **both (b) and (c)**. Option (a) is wrong because passive transducers require external excitation.
In a certain magnetic circuit, a current of one ampere in the winding produces a flux of 1 weber. If the reluctance of the magnetic circuit is doubled, then for producing the same flux, the current should be
A)
1 A
B)
½ A
C)
2 A
D)
4 A
For a magnetic circuit, flux \(\Phi = \frac{NI}{\mathcal{R}}\), where \(N\) is turns, \(I\) is current, and \(\mathcal{R}\) is reluctance. Here, the same winding is used, so \(N\) is constant. If reluctance is doubled, to keep the flux the same, the magnetomotive force \(NI\) must also double. Therefore the current must double from 1 A to 2 A. Hence the correct answer is (c) 2 A.
Two electric dipoles aligned parallel to each other and having the same axis exert a force F on each other, when a distance d apart. If the dipoles are a distance 2d apart, then the mutual force between them would be
A)
F/2
B)
F/4
C)
F/8
D)
F/16
For two electric dipoles aligned parallel along the same axis, the mutual force varies inversely as the fourth power of the separation distance, i.e. F ∝ 1/r^4. If the distance is increased from d to 2d, the force becomes F' = F × (d/2d)^4 = F/16. Hence the correct answer is F/16.
An inverse transducer converts
A)
electrical energy to any other form of energy
B)
electrical energy to light energy
C)
mechanical displacement into electrical signal
D)
electrical energy to mechanical form
An inverse transducer performs the reverse function of a normal transducer. While a transducer generally converts a non-electrical quantity into an electrical signal, an inverse transducer converts electrical energy into a non-electrical form, commonly mechanical output. Hence, the correct answer is electrical energy to mechanical form. Option (c) describes a normal transducer, not an inverse transducer.
When the plate area of a parallel plate capacitor is increased keeping the capacitor voltage constant, the force between the plates
A)
increases
B)
decreases
C)
remains constant
D)
may increase or decrease depending on the metal
For a parallel plate capacitor, the attractive force between the plates is F = \(\tfrac{1}{2}\) \(\varepsilon A E^2\). Since the voltage is kept constant, the electric field is \(E = V/d\), so F = \(\tfrac{1}{2}\) \(\varepsilon A (V/d)^2\). Thus, with voltage constant, the force is directly proportional to the plate area A. Therefore, increasing the plate area increases the force between the plates. The other options are incorrect because the force does not remain constant, decrease, or depend on the metal in this ideal capacitor case.
Very small resistance can be measured more accurately by
A)
resistance box
B)
Kelvin-double bridge
C)
Anderson bridge
D)
Schering bridge
Very small resistances are measured accurately using the Kelvin double bridge because it eliminates the effect of lead and contact resistances. A resistance box is not suitable for precise measurement of very low resistance. Anderson bridge is used for measuring inductance, and Schering bridge is used for measuring capacitance and dielectric loss.
The induced e.m.f. is maximum if the conductor cuts the magnetic field at
A)
0°
B)
45°
C)
90°
D)
none of these
The induced e.m.f. in a conductor moving in a magnetic field is proportional to vB sinθ, where θ is the angle between the direction of motion and the magnetic field. This becomes maximum when sinθ = 1, i.e. at θ = 90°. At 0°, the conductor moves parallel to the field and no flux is cut, so induced e.m.f. is zero. Therefore, the correct answer is 90°.
At a certain frequency, a uniform plane wave is found to have established a wavelength λ in a good conductor. If the source frequency is doubled, the wavelength would change to
A)
λ/√2
B)
√2 λ
C)
2 λ
D)
4 λ
For a good conductor, the propagation constant gives wavelength proportional to 1/\sqrt{f}. Thus, if the source frequency is doubled, the wavelength becomes 1/\sqrt{2} times the original value. Therefore, the new wavelength is λ/\sqrt{2}. Options (b), (c), and (d) are incorrect because wavelength decreases, not increases, when frequency increases in a good conductor.
Which one of the following is an active transducer?
A)
strain gauge
B)
selsyn
C)
photovoltaic cell
D)
photo emissive cell
An active transducer is one that generates its own electrical output without requiring an external power source. A photovoltaic cell converts light directly into electrical voltage/current, so it is an active transducer. A strain gauge is a passive transducer because it needs external excitation. A selsyn is an electromechanical device, not typically classified as an active transducer in this context. A photo emissive cell also generally requires external biasing and is not the standard answer here.
The value of Zener current is
A)
limited by external circuit resistance
B)
determined by Zener voltage
C)
totally independent of temperature
D)
always in µA range
In a Zener diode regulator, the Zener current is controlled by the series/external circuit resistance. This resistance limits the current flowing through the diode. It is not fixed solely by the Zener voltage, not totally independent of temperature, and it is not always in the µA range because it can vary from microamps to much higher values depending on the circuit.
In d.c. choppers, the waveforms for input and output voltages are respectively
A)
discontinuous and continuous
B)
both continuous
C)
both discontinuous
D)
continuous and discontinuous
In a d.c. chopper, the input is a constant d.c. source, so the input voltage is continuous. The output voltage is obtained by switching the source on and off, so it appears as a chopped waveform and is discontinuous. Hence, the correct pair is continuous and discontinuous. Option (a) reverses the order, while (b) and (c) are incorrect because the input is not discontinuous.
Out of the following three memories, which are of the combinational logic type? 1. Read only memory 2. Random access memory 3. Shift register memory
A)
1 and 2
B)
2 and 3
C)
1 and 3
D)
1, 2 and 3
Combinational logic circuits produce outputs based only on present inputs, without memory of past states. In digital electronics, ROM is treated as a combinational memory because its output depends on the addressed input lines. Shift register memory is also based on combinational logic with storage elements arranged to shift data. RAM, however, is a sequential memory because it uses storage cells and retains state. Therefore, the correct combination is 1 and 3.
After completing the execution, microprocessor returns to
A)
halt state
B)
fetch state
C)
execute state
D)
interrupt state
A microprocessor works in a cycle: fetch, decode, execute, and then repeats. After completing the execution of an instruction, it returns to the fetch state to get the next instruction from memory. Hence, the correct answer is fetch state. The other options are incorrect because halt state stops operation, execute state is the stage just completed, and interrupt state occurs only when an interrupt is serviced.
Multistage amplifiers are used in order to achieve greater
A)
voltage amplification
B)
power gain
C)
frequency response
D)
all of these
Multistage amplifiers are cascaded to obtain higher overall gain than a single stage can provide. By combining stages, they can achieve greater voltage amplification, greater power gain, and improved/desired frequency response characteristics. Hence, the correct answer is 'all of these'.
The extremely high input impedance of a MOSFET is primarily due to the
A)
absence of its channel
B)
negative gate-source voltage
C)
depletion of current carriers
D)
extremely small leakage current of its gate capacitor
A MOSFET has an insulated gate separated from the channel by a thin oxide layer. Because of this insulation, the gate draws almost no current, so the input impedance is extremely high. Hence the high input impedance is primarily due to the extremely small leakage current of its gate capacitor. The other options do not explain the input impedance: absence of channel is not generally true for all operating conditions, negative gate-source voltage is not the main reason, and depletion of current carriers relates to device operation, not input impedance.
In single pulse modulation of PWM inverter, third harmonic can be eliminated if pulse width is equal to corresponding channel capacity will be nearly
A)
30°
B)
60°
C)
120°
D)
150°
In single-pulse PWM, the harmonic content depends on the pulse width. Third harmonic elimination occurs when the pulse width is adjusted so that the 3rd harmonic component becomes zero. For a symmetrical single pulse, this happens when the pulse width is about 120°. Hence the correct option is (c). The other values do not correspond to the standard third-harmonic elimination condition in single-pulse modulation.
In the forward region of its characteristic, a diode appears as
A)
an ON switch
B)
an OFF switch
C)
a high resistance
D)
a capacitor
In forward bias, a diode conducts current with very low resistance, so it behaves like a closed or ON switch. In reverse bias, it blocks current and behaves like an OFF switch. Hence, option (a) is correct. Options (b), (c), and (d) do not describe the forward-region behavior of an ideal diode.
The number 1000 in binary is equivalent to decimal number
A)
one thousand
B)
eight
C)
four
D)
sixteen
In binary, 1000 means 1 × 2^3 + 0 × 2^2 + 0 × 2^1 + 0 × 2^0 = 8 in decimal. So the correct answer is eight. The other options are not equal to 1000₂.